Noise suppressing apparatus capable of preventing deterioration in high frequency signal characteristic after noise suppression and in balanced signal transmitting system

Abstract: A noise suppressor has a signal detecting device 1 for detecting signals A and B on signal lines, a sum signal generating device 21 for summing the detected signals, a difference signal generating device 22 for differencing the detected signals, a noise canceling device 3 for suppressing or removing noise components in a difference signal generated by the difference signal generating device 22 and a signal separating device 4 for separating the difference signal with the noise components suppressed or removed and the sum signal in signals A' and B' corresponding to the signal lines A and B.

Description: The present invention refers to a noise suppresser for use in a signal transmission system of an asymmetrically balanced type, as set forth in the preamble of claim 1. A noise suppresser of this kind is known from JP-A-59047857. The afore-mentioned document discloses a system for inserting an additional circuit in a stereo modulator and demodulator. To reduce noise of a sum signal or a difference signal at a low frequency stage, an additional circuit such as a noise reduction circuit is inserted to either a sum signal line or a difference signal line between a matrix circuit and a de-matrix circuit. The matrix circuit receives the left and right signals and outputs the sum signal and the difference signal. The de-matrix circuit receives the sum signal and the difference signal and outputs individually the left and right signals. The additional circuit, such as a noise reduction circuit, is inserted to either line between the matrix and de-matrix circuit. The left and right signals are switched at switching circuits and superimposed and outputted as a carrier suppression amplitude modulation signal forming a composite signal for main carrier modulation. Thereby, the noise in the difference signal at the low frequency stage is reduced by the additional circuit. Conventionally, in a transmission system for transmitting signals to plural signal lines, for example, a transmission system having two signals, R (right) and L (left), of a stereo signal, for suppression or removal of noise components in signals, a noise suppressor has been provided for R and L signals, respectively, to suppress or remove the noise components. As such noise suppressor, a noise limiter for simply limiting a noise level of a signal level or a higher level, a noise suppressor for dividing a signal to plural frequency bands, determining noise components in the signal, suppressing the noise components by providing a filter coefficient for cutting out the noise components to a bandpass filter (BPF) that is capable of controlling a bandpass characteristic for each of the frequency bands, and summing the frequency components thereafter and the like are used. These noise suppressors are also applied to a transmission system of balanced type in relation to noises. However, it is a problem of such noise suppressing means that a sufficient noise suppressing effect cannot be obtained, when a noise component is low, because it is difficult to determine the noise component in a signal. In consideration of such problem of conventional noise suppressor, it is an object of the invention to provide a noise suppressor capable of obtaining a sufficient noise suppressing effect. Meanwhile on the other hand in general, when a signal is recorded on a magnetic tape, or reproduced from the recorded tape in a recording/reproducing apparatus such as a video deck and an audio deck, noise is produced in the recorded signal, or the reproduced signal due to various reasons. Since this noise may deteriorate the picture (image) quality of the reproduced video signal in the video deck, and the sound quality of the reproduced audio signal in the audio deck, such noise should be removed as much as possible. Conventionally, various noise removing methods have been utilized by which noise components contained in signals are suppressed, or removed. In one conventional noise removing method, the level of the signal which has passed through the high-pass filter, is limited by the noise limiter. Then, the signal which has passed through this noise limiter are recognized as the noise component, and therefore the polarity of this signal is inverted. Finally, the polarity-inverted signal is superimposed with the original signal to cut off the noise component. In another conventional noise removing method, the signal is subdivided into a plurality of frequency bands, and a judgement is made whether or not noise components are contained in the subdivided signal. Based upon this judgement result, the filter coefficients capable of cutting such noise components are supplied to the band-pass filters (BPF) whose pass-band characteristics can be controlled, and which are provided for the respective frequency bands, thereby suppressing the noise components contained in the original signal. Thereafter, the respective frequency components are summed with each other. However, since the high frequency characteristics of the signals which have been noise-suppressed in the above-explained noise suppressing method will be deteriorated, articulation cannot be improved. Moreover, there is another problem that noise would be newly produced during the noise suppression. SUMMARY OF THE INVENTION A noise suppressor of the present invention is defined by the features of claim 1. Preferred embodiments of the invention are subject matter of the dependent claims. BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a block diagram of a noise suppressor according to a first embodiment of the invention. Fig. 2 is a block diagram of a noise suppressor according to a second embodiment of the invention. Fig. 3 is a block diagram of a noise suppressor according to a third embodiment of the invention. Fig. 4 is a block diagram of a noise suppressor according to a fourth embodiment of the invention. Fig. 5 is a block diagram of a noise suppressor according to a fifth embodiment of the invention. Figs. 6(a), (b) and (c) are drawings showing noise patterns in relation to input signal levels. Fig. 7 is a block diagram of a noise suppressor according to a sixth embodiment of the invention. Fig. 8 is a block diagram of a noise suppressor according to a seventh embodiment of the invention. Fig. 9 is a block diagram of a noise suppressor according to a eighth embodiment of the invention. Fig. 10 is a block diagram of a noise suppressor according to a ninth embodiment not part of the invention, and Fig. 11 is a drawing showing the operation of noise canceler control means of the ninth embodiment. PREFERRED EMBODIMENTS By referring to the drawings, embodiments of the invention are described below. Fig. 1 is a block diagram of noise suppressor according to a first embodiment of the invention. In detail, the noise suppressor comprises signal detecting means 1 for detecting signals, for example, signals A and B on plural (two, in this case) signal lines, and an output of the signal detecting means 1 is connected to a signal mixing unit 2 consisting of sum signal generating means 21 for generating a sum signal from the signals A and B and difference signal generating means 22 for similarly generating a difference signal. The output of the difference signal generating means 22 is connected to noise canceling means 3 for suppressing or removing noise components in the difference signal, and the output of the noise canceling means 3 and that of the sum signal generating means 21 are connected to signal separating means 4 for separating signals from the sum and difference signals (signal A' corresponding to signal A, and signal B' to signal B). Now, operation of the noise suppressor of the embodiment is described. Firstly, a signal transmission system shown in Fig. 1 is of a balanced type in relation to noises, and it is assumed that a signal A contains a noise ea, and the other signal B-contains a noise eb. Then, signals detected by the signal detecting means 1 is expressed as signals (A + ea) and (B + eb). Succeedingly, the signals are mixed by the signal mixing unit 2. In other words, a sum signal (A + B) + (ea + eb) is generated by the sum signal generating means 21, and a difference signal (A - B) + (ea - eb) by the difference signal generating means 22. However, in the case of a balanced type transmission system, because absolute values of the noises ea and eb are approximate, and phases thereof are inverse to each other, a noise component (ea + eb) of the sum signal approximates 0, and the noise is removed thereby. Therefore, the output signal from the sum signal generating means 21 is expressed as (A + B). On the other hand, since the noise component (ea - eb) of the difference signal is a noise of two times the level of ea or eb, and determination of the noise component is facilitated, the noise can be easily suppressed or removed. Now, the difference signal (A - B) + (ea - eb) is regarded as a difference signal (A - B) + 2ea, which is inputted to the noise canceling means 3. In the noise canceling means 3, the noise component 2ea is cancelled, and a difference signal (A - B)' is outputted (the signal is referred-to by (A - B)' here, because the signal components are slightly changed, when the noise is cancelled). The sum and difference signals with the noise components suppressed or removed is inputted to signal separating means, in which they are separated to signals A' and B' corresponding to the input signals, respectively. Although the embodiment was described for suppression or removal of noises in a signal transmission system of balanced type in relation to noise components, it is not limited thereto, and can be arranged so as to suppress or remove a different signal of signals in a balanced type signal transmission system. Fig. 2 is a block diagram of noise suppressor according to a second embodiment of the invention. In the figure, the embodiment is different from the first embodiment in Fig. 1 only in that a noise memory 5 storing noise patterns is connected to noise canceling means 3, and other structures are identical to those of Fig. 1. Therefore, the noise canceling means 3 makes use of the noise patterns stored in the noise memory 5 for suppressing or removing noise components contained in a difference signal. Other principal functions are identical to those of the first embodiment. Fig. 3 is a block diagram of noise suppressor according to a third embodiment of the invention. In the figure, the embodiment is different from the second embodiment shown in Fig. 2 only in that noise canceler control means 6 is connected to noise canceling means 3, and other structures are identical to those of Fig. 2. The noise canceler control means 6 outputs a parameter that is used by the noise canceling means 3 for suppressing or removing noise components. Therefore, the noise canceling means 3 makes use of the noise patterns stored in the noise memory 5 and the parameter outputted from the noise canceler control means 6 for suppressing or removing noise components contained in a difference signal. In addition, a dotted line connecting the noise canceler control means 6 and noise memory 5 shows a modified embodiment that is arranged so as to update the noise patterns in the noise memory 5 according to the parameter outputted from the noise canceler control means 6. Principal functions are identical to those of the first embodiment. Fig. 4 is a block diagram of noise suppressor according to a fourth embodiment of the invention. In the figure, the embodiment is different from the third embodiment shown in Fig. 3 in that receiver means 7 for receiving electric wave signal to obtain a signal that is inputted to signal detecting means, and receiving condition detecting means 8 for detecting a receiving condition of a signal received by the receiver means 7 are provided. An output of the receiving condition detecting means 8 is connected to an input of noise canceler control means 6. The noise canceler control means 6 outputs a parameter to noise canceling means 3 according to the receiving condition detected by the receiving condition detecting means 8, and updates noise patterns in noise memory 5. Here, the electric wave signal is, for example, a stereo broadcast wave signal, and the receiving condition is an input signal level, field strength of the received electric wave or the like. Fig. 5 is a block diagram of noise suppressor according to a fifth embodiment of the invention. The noise suppressor of the embodiment is an application example to a receiver for FM stereo broadcast wave. In Fig. 5, the noise suppressor is provided with an FM receiving unit 9 for receiving an FM stereo broadcast wave, and separating the stereo signal received to right and left channel signals (hereinafter referred to as R and L signals), the FM receiving unit 9 is connected with field information detecting means 10 for detecting field information of the received signal, and the R and L signal outputs are supplied to an analog-to-digital converter 11 for converting an analog to a digital signal. The analog-to-digital converter 11 is connected to a signal mixing unit 2 for generating sum and difference signals from the R and L signals, and the difference signal output is connected to noise canceling means 3 for suppressing or removing noise component in the difference signal. The signal mixing unit 2 includes sum signal generating means consisting of an adder 23 and a fast Hartley transducer (hereinafter referred to as FHT) 25 for generating the sum signal and difference signal generating means consisting of a subtracter 24 and FHT 26 for generating the difference signal. An output (sum signal) of the signal mixing unit 2 and that (difference signal with noises cancelled) of the noise canceling means 3 are connected to signal separating means 4 for separating R' signal from L' signal that correspond to R and L signals according to the sum signal and difference signal with noises cancelled, and an output of the signal separating means 4 is connected to a digital-to-analog converter 12 for converting a digital to an analog signal. The signal separating means 4 includes a circuit consisting of an adder 43 and an inverted fast Hartley transducer (hereinafter referred to as IFHT) 45 for generating the R' signal and a circuit consisting of a subtracter 44 and IFHT 46 for generating the L' signal. On the other hand, the field information detector 10 is connected to the noise canceler control means 6 that outputs a parameter used for suppressing or removing noise components by the noise canceling means 3. Additionally, a noise memory 5 for storing noise patterns is connected to the noise canceling means 3. Here, as shown by a dotted line similarly to Fig. 4, the components may be arranged such that the noise patterns are updated by the parameter. The FM receiving unit 9 provides receiver means, and the field information detecting means 10 forms receiving condition detecting means. Now, a processing method of the noise canceling means 3 according to the fifth embodiment is illustratively described. Here, subtraction of the noise pattern is executed for each frequency component in each predetermined bandwidth. As shown in Fig. 17, firstly, a power spectrum |X(k)| 2 of an output H(k) that is subjected to a fast Hartley transduction by the FHT 26 is calculated by a formula (Formula 1) below (step S1). Formula 1 X(k) 2 = (H 2 (k) + H 2 (N - k)) / 2 Succeedingly, by using a formula (Formula 2) below, a square root of the power spectrum obtained is calculated (step S2). Formula 2 x = -0.1985987 x 2 + 0.8803385 x +0.3175231 (accuracy of 10 bits) Then, according to the value calculated and a noise pattern |W(k)| outputted from a noise generating unit (noise memory 5 in Fig. 5), a noise spectrum is subtracted by using a formula (Formula 3) below (step S3). Formula 3 S(k) = H(k) * (1 - α W(k) / X(k) ) S(N - k) = H(N - k) * (1 - α W(k) / X(k) ) However, correction should be performed to make (1 - α W(k) / X(k) ) ≧ β Setting of α and β α : 0.5 (for an extremely weak field) 0.9 (for a strong field) β : 0.2 to 0.6 where α is a canceling coefficient that determines a subtraction amount for subtraction of a noise pattern, and β a clamping coefficient for controlling the canceling coefficient used in cancellation of a noise to minimize adverse effects of removing the noise. In succession, a principle of noise suppression of the fifth embodiment is described. When right and left channel signals including noises are referred to as R' and L', those without noises as R and L, and the noises as E R , E L , respectively, Formula 4 R' = R + E R Formula 5 L' = L + E L At this point, (R + L') and (R' - L') are considered. Formula 6 A = (R' + L') / 2 = (R + L) / 2 + (E R + E L ) / 2 Formula 7 B = (R' - L') / 2 = (R - L) / 2 + (E R - E L ) / 2 As a result of auditory experiment, it was confirmed that a noise level of signal A is extremely low, and that of signal B extremely high. Therefore, when E R + E L ≒ 0, Formula 8 E R ≒ -E L And from (Formula 6), (Formula 7) and (Formula 8), the following formula is obtained: Formula 9 A ≒ (R + L) / 2 Formula 10 B ≒ (R - L) / 2 - E L ≒ (R - L) / 2 + E R Succeedingly, by suppressing noises in the state of (Formula 10), (E L ', E R '), A + B and A - B are obtained. Formula 11 A + B = (R + L) / 2 + (R - L) / 2 - E L ' = R - E L ' = R + E R ' Formula 12 A - B = (R + L) / 2 - ((R - L) / 2 - E L ') = L + E L ' In (Formula 11) and (Formula 12), noises are reduced comparing with (Formula 4) and (Formula 5). Now, the noise suppression is described in detail. When signals and noises after the noise suppression are referred to by R S , L S , E RS , E LS , respectively, in correspondence with the signals R, L and noises E R , E L , (Formula 7) is transformed into the following formula (Formula 13), after it is subjected to the noise suppression. Formula 13 B' = (R S - L S ) / 2 + (E RS - E LS ) / 2 Succeedingly, A + B' and A - B' are obtained. Formula 14 A + B' = (R + R S ) / 2 + (L - L S ) / 2 + (E R + E RS + E L - E LS ) / 2 Formula 15 A - B' = (R - R S ) / 2 + (L + L S ) / 2 + (E R - E RS + E L + E LS ) / 2 When E R = -E L ; R S = R - R SD (R SD : Error cancelled); and L S = L - L SD (L SD : Error cancelled), Formula 16 A + B' = R - R SD / 2 + L SD / 2 + (E RS - E LS ) / 2 = R + (L SD - R SD ) / 2 + (E RS - E LS ) / 2 Formula 17 A - B' = L - (L SD - R SD ) / 2 - (E RS - E LS ) / 2 Then, R and L channel signals are independently subjected to the noise suppression. Formula 18 R S ' = R - R SD ' + E RS ' Formula 19 L S ' = L - L SD ' + E LS ' By comparing between (Formula 16), (Formula 18), (Formula 17) and (Formula 19), effects of the noise suppression are considered. It is predicted that differences of suppression error (L SD - R SD )/2, R SD' and L SD' can be correlated as in a formula below. Formula 20 (L SD - R SD ) /2 < R SD ' , L SD ' Formula 21 ∵ L - R < L , R (Generally, the formula is true, if any in-phase component is present in L and R.) It is appreciated that the left term is reduced than in the case of the clamping coefficient β = 50% (Formula 21). Therefore, the effects of signal components are reduced. Next, residual noise components of (Formula 16), (Formula 18), (Formula 17) and (Formula 19) are considered. Although it is predicted that (E RS - E LS )/2, E RS' and E LS' are correlated as: Formula 22 (E RS - E LS ) / 2 ≒ E RS ', E LS ' because effects on signal components are reduced than in the case of (Formula 20), the noise suppression level can be set at a higher level. Additionally, from (Formula 7), since the component of (E R - E L ) is higher than E R and E L , the noise component is more easily identified. Thus, when L and R are compared, many components included in the noise have phases opposite to each other. For example, by comparing (Formula 6) with (Formula 7), the noise component can be identified. Generally, |R + L| < |R - L| (when an antiphase component is absent). In the case a lot of noises are included, it comes to be as: Formula 23 A < B Therefore, by obtaining L' - R', noises with antiphase components can be identified, the noise suppression can be effectively achieved, and effects of the noise suppression by L' - R' on the original signal is reduced. Now, operation of the noise suppressor according to the fifth embodiment is described. Firstly, a stereo signal received by the FM receiving unit 9 is separated to R and L signals, and outputted to the analog-to-digital converter 11. In the analog-to-digital converter 11, R and L signals are converted to a digital signal, respectively, and inputted to the signal mixing unit 2. Here, it is assumed that the R and L signals include noise components having an absolute value approximately equal and phases opposite to each other. Then, the R and L signals are summed in the adder 23, the noise components are consequently offset by each other, and a sum signal (R + L) is produced. After that, the sum signal (R + L) is subjected to a fast Hartley transduction by the FHT 25 for easier processing of a noise pattern. On the other hand, subtraction between the R and L signals is executed in the subtracter 24, the noise components are approximately doubled as a result, and a difference signal (R - L) with two times the noise components is produced. Then, the difference signal (R - L) with two times the noise components is subjected to a fast Hartley transduction by the FHT 26. Here, H(k) that is an output of the FHT is calculated by using a formula (Formula 24) below. Succeedingly, the sum signal (R + L) is inputted to the signal separating means 4, as the noise is already removed. On the other hand, the difference signal (R - L) with two times the noise components is inputted to the noise canceling means 3 to suppress or remove the noise components. Meanwhile, the noise canceler control means 6 outputs a parameter to the noise canceling means 3 according to field information (it may simply be an input signal level) of a received signal detected by the field information detector 10. Moreover, a noise pattern corresponding to the field information is read from the noise memory 5 by the noise canceling means 3. The parameter serves for changing a subtraction rate according to the field information to minimize effects of the subtraction on the signal, and allow the noise components to be suppressed, in the case the noise pattern is subtracted from the noise components included in the difference signal, for example. Furthermore, assuming the parameter is a canceling coefficient α for a parameter multiplied by the noise pattern, and the field information an antenna input level, the value of canceling coefficient α is increased, when the antenna input level is relatively high, and the value of canceling coefficient α is reduced on the contrary, when the antenna input level is relatively low. For example, by obtaining and storing in the noise memory 5 noise patterns corresponding to input signal levels as shown in Figs. 6(a), (b) and (c), a noise pattern corresponding to an input signal level detected can be obtained from the noise patterns in the memory by means of interpolation or the like. The difference signal (R - L)' (as the signal is somewhat transformed by the noise canceling means 3, it is now expressed as (R - L)') with noise components suppressed in such manner is inputted to the signal separating means 4. In the signal separating means 4, the sum signal (R + L) and difference signal (R - L)' are summed by the adder 43, and a resultant signal is, then, subjected to an inverted Hartley transduction by the IFHT 45, while subtraction between the sum signal (R + L) and difference signal (R - L)' is executed simultaneously by the subtracter 44, and a resultant signal is subjected, then, to an inverted Hartley transduction by the IFHT 46, as a result, an R' signal corresponding to the R signal is outputted from the IFHT 45, and an L' signal corresponding to the L signal from the IFHT 46. The R' and L' signals are converted to an analog signal, respectively, by the digital-to-analog converter 12. Although the fifth embodiment is arranged such that the fast Hartley transduction process is executed after the signal mixing in the signal mixing unit 2, it may be arranged so that the fast Hartley transduction process is executed, on the contrary, before the signal mixing. In addition, although the fifth embodiment is constructed so that the inverted fast Hartley transduction process is executed after the signal separation in the signal separating means 42, it may be such that the inverted fast Hartley transduction process is conducted before the signal separation, contrarily. Also, in the fifth embodiment, although the noise canceling means 3 i provided only in the difference signal system, it is not limited thereto, and may be placed in the sum signal system as well. Moreover, although the fifth embodiment utilizes input signal levels as the field information, and is arranged such that noise components in relation to the input signal levels are suppressed or removed, it is not limited thereto, and can be designed such that a multipath signal propagation is used as the field information, and noise components caused by the multipath signal propagation is suppressed or removed. In such case, noise patterns in relation to multipath propagation can be employed as the noise patterns. Furthermore, in the fifth embodiment, although the difference signal (R - L) is always subjected to the noise canceling, alternatively, as shown in Fig. 15, for example, a value |R + L| of the sum signal (R + L) may be compared with a value |R - L| of the difference signal (R - L), and the difference signal (R - L) is subjected to the noise cancellation (step S2), consequently, when the value of difference signal (R - L) is higher than the other, while the sum signal (R + L) is multiplied by a predetermined constant, 0.5, for example, when the value of sum signal (R + L) is higher, and the value k x |R + L| is compared with that of |R - L| (step S3). As a result of the comparison, the difference signal (R - L) is increased, or the sum signal (R + L) is subjected to the noise cancellation (step S4), if the value k x |R + L| is higher, while the multiplied value is left as it is, if the value k x |R + L| is lower, on the contrary. Fig. 7 is a block diagram of noise suppressor according to a sixth embodiment of the invention. The noise suppressor of the embodiment is provided, for example, between the output of signal separating means 4 and input of digital-to-analog converter 12 of Fig. 5 for one or both signals. In Fig. 7, the noise suppressor is provided with signal noise canceling means 13 for suppressing or removing noise components in a signal input from the signal separating means and a mixer 15 for mixing an output of the signal noise canceling means 13 and the signal input from the signal separating means. The signal noise canceling means 13 is connected to noise generating means 14 for generating a noise according to field information, and the mixer changes the mixing ratio between the two signals according to the field information. Here, the field information is a multipath signal propagation information, and a noise component in the signal output from the signal separating means is suppressed by outputting a noise pattern from the noise generating means 14 to the signal noise canceling means 13 according to the multipath signal propagation. The noise generating means 14 is, for example, a noise memory storing noise patterns in relation to multipath signal propagation. The mixer 15 mixes the signal from the signal separating means and a signal with its noise component suppressed at the mixing ratio according to the multipath signal propagation. Regarding the mixing ratio, the ratio of noise-suppressed signal is increased in an area with a higher intensity of multipass, and the ratio of signal from the signal separating means is increased in an area with a lower intensity of multipath propagation. Fig. 8 is a block diagram of noise suppressor according to a seventh embodiment of the invention. In the embodiment, a noise memory consists of a static characteristic noise memory 51 storing noise patterns of static noises and a multipath signal propagation noise memory 52 storing patterns of noises caused by a multipath propagation, and suppression of noise components by noise canceling means 3 is executed according to a receiving condition (antenna input level, in this case) detected by receiving condition detecting means 8 in such manner that the static characteristic noise memory 51 is selected by noise selecting means 50, if an antenna input level is low, while the multipath signal propagation noise memory 52 is selected, if the antenna input level is high, and the noise patterns in the noise memory selected are utilized for the suppression. A reference of the antenna input level is approximately 30 dB µ, for example, in the case of an FM receiver. Principal functions are similar to those described above. Fig. 9 is a block diagram of noise suppressor according to an eighth embodiment of the invention. In the embodiment, a noise memory comprises a monaural noise memory 53 storing noise patterns of noise components contained in an (L + R) signal, a stereo noise memory 54 storing noise patterns of noise components in an (L - R) signal and a multipath signal propagation noise memory 55 storing patterns of noises caused by a multipath. In addition, noise canceling means 3 is provided in either systems for R and L signals outputted from a stereo demodulator (not shown) of an FM receiving unit 9. Suppression of noise components by the noise canceling means 3 is conducted by switching between the noise memories according to field information (antenna input level, in this embodiment) detected by a field information detector 10 to select an optimum noise pattern by memory switching means 60. The switching condition is such that the monaural noise memory 53 is selected, if a stereo pilot signal output from the stereo demodulator is absent (that is, resulting in a monaural signal), or the antenna input level is low; the stereo noise memory 54 is selected, if the stereo pilot signal output is present, and 19 kHz and 38 kHz signals are not included in the antenna input, and the multipath signal propagation noise memory 55 is selected, if the stereo pilot signal output is present, and 19 kHz and 38 kHz signals are included in the antenna input. Fig. 10 is a block diagram of noise suppressor according to a ninth embodiment not part of the invention. In the embodiment, a receiving condition (intermediate frequency signal level, in this case) is detected from an intermediate frequency signal of an FM receiving unit 19 by intermediate frequency level detecting means 91, and a stereo pilot signal (19 kHz component and a double frequency component) from the intermediate frequency signal is detected by pilot signal detecting means 92. The level of intermediate frequency signal (IF) and the stereo pilot signal (SP) detected are determined for a signal condition by signal determiner 93, and inputted to noise canceler control means 6. The noise canceler control means 6 outputs a parameter to noise canceling means 3, according to a result of the determination. As shown in Fig. 11, the noise canceler control means 6 sets one of parameters, that is, a canceling coefficient α to low to intermediate, if the determination result of the signal determiner 93 indicates that the level of intermediate frequency signal (IF) is high, and the stereo pilot signal (SP) is low (Fig. 11-A), intermediate, if the IF level is high, and the SP signal is high (Fig. 11-B), low, if the IF level is low, and the SP signal is low (Fig. 11-C), and high, if the IF level is low, and the SP signal is high (Fig. 11-D).

Claims: A noise suppressor for use in a signal transmission system of an asymmetrically balanced type, comprising: signal detecting means (1) for detecting signals (A, B) on plural signal lines, sum signal generating means (21) for summing the signals (A, B) detected by the signal detecting means (1), difference signal generating means (22) for differencing the signals (A, B) detected by the signal detecting means (1), and noise canceling means (3) for suppressing or removing noise components (ea - eb) in the difference signal, characterized by , further comprising: signal separating means (4) for separating the signals (A', B') corresponding to the signals (A, B) from the signal lines with the noise components suppressed or removed from said difference signal, and a noise memory (5) for storing noise patterns to be used in suppression or removal of the noise components by the noise cancelling means (3). A noise suppressor according to claim 1, comprising: a noise canceler control means (6) for outputting parameters to be used in suppression or removal of the noise components by the noise cancelling means (3). A noise suppressor according to claim 2 wherein: the noise patterns in the noise memory (5) are updated by the parameters. A noise suppressor according to claim 2 or 3, comprising: receiver means (7) for receiving signals to obtain the signals on said signal lines and receiving condition detecting means (8) for detecting a receiving condition of the receiver means (7), wherein the noise canceler control means (6) outputs the parameters to the noise canceling means (3) according to the receiving condition detected by the receiving condition detecting means (8). A noise suppressor according to claim 4, wherein the receiving condition detecting means (8) detects the level of a received signal, and the noise canceler control means (6) outputs such parameter that reduces the noise pattern, when the detected level is relatively low. A noise suppressor according to claim 4, comprising: an analog-to-digital converter (11) for converting an analog signal received by the receiver means (7) to a digital signal and a digital-to-analog converter (12) for converting a digital signal outputted by the signal separating means (4) to an analog signal, wherein the suppression or removal of the noise components is achieved in the digital signal domain. A noise suppressor according to claim 4 or 6, wherein the receiver means (7) is a receiver for receiving a stereo signal, and the signal detecting means (1) detects L and R signals of the stereo signal. A noise suppressor according to claim 7, wherein the difference signal (R - L) is subjected to the suppression or removal of a noise unless | R + L | > | R - L |, otherwise, (R + L) is subjected to the suppression or removal of the noise, (R - L) is increased, or the suppression or removal of the noise is unexecuted. A noise suppressor according to claim 4, 6 or 7, comprising: multipath signal propagation detecting means (10) for detecting a multipath signal propagation from a signal received by the receiver means (7) and signal noise canceling means (3) for suppressing or removing noise components contained in output signals from the signal separating means (4), respectively, wherein the signal noise canceling means (3) suppresses or removes the noise components according to the detected multipath signal propagation noise. A noise suppressor according to claim 9, comprising: a mixer (15) for mixing an output signal of the signal noise canceling means (13) and the signals according to the multipath propagation, to provide an output. A noise suppressor according to claim 4, 6, 7 or 9, wherein the noise cancelling means (3) comprises band dividing means for dividing a signal to predetermined frequency bands and a noise cancelling unit (3) provided for each of the divided bands. A noise suppressor according to claim 11, for changing subtraction parameters according to a frequency band and multipath signal propagation in noise suppression or removal using the spectral subtraction method. A noise suppressor according to claim 4, wherein the noise memory comprises a static characteristic noise memory (51) for storing noise patterns of static noises and a multipath signal propagation (52) for storing noise patterns of noises caused by a multipath signal propagation, and the receiving condition detecting means (8) detects the level of a received signal, and comprises: noise selecting means (50) for selecting the noise pattern of the static characteristic noise memory (51), if the detected level of received signal is at a predetermined value or a lower value, and the noise pattern of the multipath signal propagation noise memory (52), if the level of received signal is at the predetermined value or a higher value. A noise suppressor according to claim 7, wherein the noise memory comprises plural memories (53, 54, 55) for storing different noise patterns, respectively, and memory switching means (60) is provided for switching between the plural memories (53, 54, 55) according to the receiving condition detected by the receiving condition detecting means (10) and a stereo pilot signal received by the receiver means (7). A noise suppressor according to claim 14, wherein the different noise patterns are of noises contained in the (L + R) signal that is produced by the L and R signals, those in the (L - R) signal, and the noises caused by the multipath signal propagation. A noise suppressor according to claim 7, comprising: intermediate frequency level detecting means (91) for detecting a receiving condition from an intermediate frequency signal that is outputted by the receiver means (9) and pilot signal detecting means (92) for detecting the stereo pilot signal from the intermediate frequency signal, wherein the noise canceler control means (6) outputs a parameter according to the detected receiving condition and stereo pilot signal.

Inventors: KANE JOJI, NOHARA AKIRA, KANE, JOJI, NOHARA, AKIRA
Applicants: MATSUSHITA ELECTRIC IND CO LTD, MATSUSHITA ELECTRIC INDUSTRIAL CO., LTD.
Application year: 1994
Main classification: H04B1
IPC codes: H04B1, G11B20